Mechanistic Investigation of Thermal and Photoreactions between Boron and Silane.

Density functional theory and high-level ab initio calculations were performed to elucidate the detailed reaction mechanism from B and SiH to a structure with two bridging H atoms (Si(μ-H)BH, silicon tetrahydroborate). On the basis of the calculated results, this reaction mechanism includes both thermal and photochemical reactions. Especially, thermal conversion of silylene dihydroborate (HB═SiH) to Si(μ-H)BH is not feasible because two high energetic barriers must be overcome. In contrast, the reverse reaction is feasible because it is effectively only necessary to overcome a single barrier. The characteristics of the excited states of HB═SiH and Si(μ-H)BH have been identified. Two successive conical intersections (CIs) are involved in the photochemical reaction. The BSiH bending coordinate is almost parallel to the reaction coordinate near the regions from the second CI to Si(μ-H)BH. The activated BSiH bending mode lift the degeneracy of the second CI, thereby the reaction readily proceeds to Si(μ-H)BH. All calculated results in this work reasonably well describe the recent experimental observations.